People, A couple of times I suffered of unneeded strictness in Haskell. And now unneeded laziness bites. Consider the following program: ------------------------------------------------------- main = let mK = <formed in a certain complex way> :: Map.Map Partition Integer row = [0, 1, 0 ... 0] (mK', p) = decompose mK row infoK' = sum $ map sum $ mapmap (\ n -> rem n 2) $ Map.elems mK' in putStr (shows p -- seq infoK' "\n") ------------------------------------------------------- In ghc-6.6, it needs at least 100 Mb of heap to perform. If I set seq infoK' between `shows' and `p', then it suffices 14 Mb. If I set there seq mK', then again it needs 100 Mb. 7 times as large. mK is a triangular n by n matrix, n is about 900 (so, there are about 1 million of elements in mK). mK has small integers in it. p is found actually by solving the linear system X x mK = row. Now: how to program real problems in Haskell ? Should I try to set seq everywhere and see whether it helps ? Also note: seq mK' was not sufficient. I do not know, probably, as mK' has structure, seq does not evaluate all its levels. Therefore the programmer needs to invent a function that gathers much of information of mK' and puts it into one Integer (infoK' = sum ... mK'). Then, seq infoK' forces most of evaluation of mK'. Is this a generic style to program real problems ? To write all this strange code instead of plain let (_, p) = decompose mK row in putStr (shows p "\n") ? The story was as follows. My friend asked me to decompose certain large symmetric polynomial into elementary symmetric ones. This was needed for a real mathematical problem. I applied my DoCon program, which is written in Haskell. 1000 Mb RAM was not enough. I started to investigate, because I suspected that this case needs several times smaller memory. I found a fragment of the type shown above. And I never use `seq'. I have put a bit smaller example to make it 300 Mb sufficient. The effect was discovered by occasion. I have forgotten that `decompose' returns a pair with a large matrix as fst and wrote let p = decompose mK row in putStr $ show p It has computed in a relatively small memory, and fast, but before the p value, it printed several pages of the matrix value. I said "Oh! we need to improve this, we only need p, the second part": let (_, p) = decompose mK row in putStr $ show p And this takes 7 times more of memory and 2 times more of time! Then, I understood, what is happening and verified the idea by introducing seq. And now, I wonder ... Thank you in advance for your help and comments. Regards, ----------------- Serge Mechveliani mechvel@botik.ru